1. Field of the Invention
The present invention relates to a receiver-transmitter device for transmitting data by means of focused, modulated light beams, more particularly, by means of pulse-coded laser beams, comprising a photoelectric sensor intended for the incoming light beam and coupled to an optical receiver device, an optical device for transmitting an answer light beam in the direction of the input light beam and a modulator for modulating the answer light beam. As used herein, the term light beam includes visible and invisible beams such as infrared and other optical radiation beams which may be optically controlled.
2. Description of the Prior Art
The exchange of data between two stations by means of a modulated light beam can occur, for example, in conventional communication systems such as telephoto, modulated light speech, teleprinting systems, etc. A preferred application for the present invention is that of data exchange in military identification systems for detecting and distinguishing targets, known under the abreviation IFF. In systems of this type, such as the systems described in German Auslegeschrift No. 2,142,944 and German Offenlegungsschrift No. 2,215,463, an interrogating station, which is generally connected with a weapon, transmits an interrogation signal to an unidentified target, which, if it is a friendly or allied target, is received at the target, where it triggers the transmission of a suitably coded answer signal to the interrogating station. Dependent on whether this answer is or is not received, further operations are then controlled at the interrogating station, for example, the releasing of the target bombardment.
A problem which occurs not only in the case of this military data exchange system but also in the case of every data exchange system employing focused light beams, more particularly, laser beams, is that the answer light beam must be transmitted exactly in the direction of the received light beam in order for it to reach the interrogating station. The earlier proposals for solving this problem were directed to the task of designing the optical receiver device in such a way that it was possible to determine the direction of the incoming light beam in terms of its elevation and azimuth, thus enabling a transmitter for transmitting the answer light beam to be controlled according to the determined directional values. As this system necessitates the use of complicated direction finding systems and directional coupling devices for guiding the transmitting device even in the case of stationary receiver stations, it will be appreciated that in the case of receiver stations mounted on transport means, for example, on tanks, ships, etc., the problem is even more difficult and can only be solved at great expense using gyroscopically stabilized devices. Furthermore, these systems do not offer a sufficiently high degree of operational reliability when they are used as IFF equipment; this demands immediate reaction of the target, generally in the msec. range. The orientation periods required by the regulating devices would be excessive. An answer signal transmitter which transmits over a wide optical range is not desirable, as the signals could be picked up at other points. Furthermore, it would be nceessary to use considerably more energy which should be avoided in view of the threshold value which the human eye can tolerate.